Like BeaG said, good SEO and marketing are the best way to be seen and get sales. I seldom renew anything. I usually only renew after an item expires (and I've revamped by updating tags, titles, descriptions, and sometimes photos on the listing). Every now and then, maybe one a week or two, I will renew an item when I see it hasn't sold in a while and normally it sells more frequently.
But that also depends on your item category. If there are many other buyer options for the same type of product as yours, and recency is more of a factor for your category, then you'd need to renew more often.